It is well known that the presence of basic nitrogen compounds in petroleum oil can deleteriously affect the performance of the subsequent catalytic processes, especially where acidic catalysts are used. For example, nitrogenous compounds present in the vacuum gas oil or residual oil can deactivate catalysts that are employed in hydrodesulfurization. A variety of chemical and physical treatments for reducing the level of nitrogen compounds in oils have been developed. Chemical methods include, for instance, (i) hydrodesulfurization/hydrodenitrogenation (HDS)/(HDN) processes and (ii) oxidation processes. HDS/HDN techniques for removing nitrogen compounds from high boiling petroleum oils are well established. Oxidation techniques, which have been developed more recently, are usually employed in combination with sulfur removal. The oxidation processes typically include an extraction or adsorption step subsequent to oxidation. Oxidation methods are described, for example, in U.S. Pat. No. 6,160,193 to Gore, U.S. Pat. No. 6,274,785 to Gore, U.S. Pat. No. 6,402,940 to Rappas, U.S. Pat. No. 6,406,616 to Rappas et al, U.S. Pat. No. 6,596,914 to Gore et al., and U.S. Patent Application Publication No. 2004/0178, 122 to Karas et al.
The most common physical techniques for removing nitrogen compounds are liquid extraction and solid adsorption which are particularly suited for treating high boiling petroleum oils. For example, U.S. Pat. No. 4,846,962 to Yao describes a method for removing basic nitrogen compounds (BNCs) from solvent extracted oils by adsorbing the BNCs to solid acidic polar adsorbents. The oils are extracted with common extraction solvents, preferably N-methyl-2-pyrrolidone (NMP). The resulting raffinate which contains the extracted oil is passed through a solid adsorption unit that contains an acidic adsorbent, such as silica-alumina, high alumina base amorphous cracking catalyst or crystalline zeolite. Depending upon the type of adsorbent and adsorption process conditions employed, the adsorbent can be regenerated by either purging with hydrogen at elevated temperatures and pressures or by washing the BNC saturated adsorbent with extractive solvent, e.g., NMP. In either case, adsorbent regeneration can be expensive.
U.S. Pat. No. 6,248,230 to Min et al. describes a solid adsorption method for removing natural polar compounds, which are predominantly basic nitrogen compounds, from hydrocarbon fractions that preferably have boiling points that range from 200 to 400° C. in advance of catalytic hydroprocessing. The process is said to significantly improve hydrotreater performance so as to produce cleaner diesel fuels with lower sulfur content. The preferred adsorbent is silica gel which is regenerated with a polar solvent, such as methanol. Similarly, U.S. Pat. No. 5,730,860 to Irvine discloses a method for treating naphtha with high concentrations of polar compounds (including nitrogen compounds) in a counter-current fluidizing adsorption process. The adsorbent is regenerated by contact with a reactivating medium such as hydrogen gas at elevated temperatures.
While adsorption can be very selective in removing nitrogen compounds from hydrocarbons, this method is not commercially feasible for a number of reasons. To begin with, implementing the technique requires a significant initial capital investment followed by substantial operating costs. The high costs are attributable, in part, to the fact that adsorption is normally a batch operation, with respect to the adsorbents, which is divided into an alternating sequence of operation and regeneration cycles. The logistics of the regenerative procedure is itself quite complex and requires complicated plant design in order to implement different fluid patterns into and out of an adsorption column as well as to reverse the flow directions at various stages during the regeneration cycle. Another reason against using adsorption is that absorbents have limited and inconsistent adsorbent capacities and lives. Using absorbents with predictable adsorbent lives is critical to the commercial success of any adsorption process. Often adsorbent life must be determined empirically for a particular application and the experiments entailed may be extensive.
The adsorption process may be suitable for removing nitrogen compounds where the nitrogen content in the hydrocarbon feed stream is extremely low, that is, in the low parts per million (ppm) or parts per billion (ppb) levels. At these minute concentrations, the process of removing nitrogen may require only infrequent adsorbent replacement and no adsorbent regeneration is needed. Since no adsorbent regeneration is required, adsorption can be advantageously based on a neutralization reaction between acid and base. Nitrogen adsorption is manifested in the form of a strong non-reversible adsorption of basic nitrogen compounds onto adsorbents with acidic sites.
With respect to prior art extraction techniques, U.S. Pat. No. 4,113,607 to Miller describes a process for upgrading hydrogenated distillate oil by extracting nitrogen compounds from the oil by liquid-liquid extraction using a solution of ferric chloride in furfural. The raffinate (oil) phase is said to be especially suitable for use as feedstock for catalytic cracking or hydroprocessing that employs an acidic catalyst. U.S. Pat. No. 4,960,507 to Evans et al. discloses a two-step extraction process for removing basic heterocyclic nitrogen from petroleum oils whereby an aqueous acidic solvent is used in a first extraction step to remove the bulk of the nitrogen compounds from the oil and an immiscible hydrocarbon solvent is used in a second extraction step to further lower the nitrogen content in the oil. Aqueous acidic solvents include carboxylic acids and halogen-substituted carboxylic acids while immiscible hydrocarbon solvents include C3 to C12 paraffins, C3 to C12 olefins and C3 to C12 ethers. U.S. Pat. No. 4,960,508 to Evans discloses a similar two-step extraction process for removing basic heterocyclic nitrogen from petroleum oils whereby an aqueous concentrated acidic solvent is used in a first extraction step to remove the bulk of nitrogen compounds from the oil and an aqueous diluted acidic solvent is used in a second extraction step to further lower the nitrogen content. The concentrated acidic solvent comprises an aqueous solution containing 85 to 95 wt % of carboxylic acids, halogen-substituted carboxylic acids and mixtures thereof while the diluted acidic solvent has the same acid mixtures as the concentrated form but at lower concentrations of about 25 to 75 wt %.
U.S. Pat. No. 4,426,280 to Chen et al. describes a two-step extraction process for removing nitrogen compounds from shale oil that employs formic acid, acetic acid, and mixtures thereof as the extraction solvents. In the initial extraction, the oil is contacted with a low acid strength solvent containing 30 to 50 wt % acids in a first extraction zone and subsequently the oil is contacted with a high acid strength solvent containing 70 to 90 wt % acids in a second extraction zone. U.S. Pat. No. 4,483,763 to Kuk et al. describes an extraction method for removing nitrogen compounds from shale oil using a three-component extraction solvent comprising an organic polar solvent, an acid and water, e.g., a mixture of furfural alcohol, hydrochloric acid and water. U.S. Pat. No. 4,169,781 to Miller describes an extraction method for removing nitrogen from coal-derived coker oil where the extraction solvent consists of a solution of ferric chloride in furfural.
Light petroleum oils that are used as petrochemical feedstocks in many catalytic processes may contain only very low levels of sulfur and nitrogen. Recent advances in catalyst technology have lead to the developed high activity catalysts that have substantially improved the productivity and economics of many of these processes. Unfortunately, these high activity catalysts are extremely sensitive to sulfur and nitrogen poison; they are particularly sensitive to basic nitrogen compounds. For example, alkylation and isomerization reactions that have been catalyzed by strong inorganic acids, such as hydrofluoric acid, sulfuric acid, and aluminum chloride slurry are now catalyzed by solid zeolitic catalysts that have very active acidic catalytic sites that are vulnerable to poison from basic nitrogen compounds in the feedstock. An example of a commercially significant alkylation reaction is that of benzene with ethylene or propylene to produce ethylbenzene or cumene, respectively. Important isomerization reactions include, for example, the production of paraxylene from othoxylene or metaxylene and the production of cyclohexane from methyl cyclopentane. In this latter synthesis, for example, the benzene feedstock must be essentially free of nitrogen compounds, preferably less than 30-100 ppb.
There is an urgent need for a cost effective, efficient process for removing nitrogen compounds from hydrocarbon to produce products such as light petroleum oils having ultra-low nitrogen content. The products are feedstock for subsequent processes that are catalyzed by catalysts that are otherwise deactivated by nitrogen compounds and particularly by basic nitrogen compounds. It is desired that the process can be continuous and operates under mild conditions.